This invention pertains generally to monitoring exhaust emissions from non-stationary sources and particularly to monitoring exhaust emissions from non-stationary diesel engines.
Diesel engines emit exhaust plumes that can contain particulate matter which is visually evident as opacity (i.e., smoke) and measurement of exhaust plume opacity, i.e., the particle density (number of particle/unit volume), is of importance in many areas. Regulatory agencies, both state and Federal, regulate moving and stationary emissions sources based, among other things, on the opacity of their emissions. Consequently, these agencies generally require periodic testing of diesel engines to ensure they continue to meet exhaust emissions requirements. Further, the opacity of exhaust plumes can be useful as an indicator of proper engine function. Thus, exhaust opacity measurements are important to diesel engine manufacturers, service technicians and diesel engine owners to help ensure that an engine is being properly maintained and tuned.
In general, opacity measurement methods for exhaust plumes can be divided into two broad categories; 1) full flow measurement and 2) partial flow measurement. The typical full flow measurement system consists of a beam of light from a light source that traverses an exhaust plume to a detector. The source and detector are generally affixed to a rigid frame or fixture which is, in turn, either mounted on the emission source or fixedly attached to some arrangement which allows the source to be positioned such that the diesel exhaust plume can pass undisturbed between the light source and the detector. There are numerous problems associated with this method of exhaust plume opacity measurement, having generally to do with accurate determination of the center of the plume with respect to the light source and detector.
In the partial flow measurement system for determining opacity of an exhaust plume, a portion of the exhaust plume is diverted by a probe and hose assembly into a sampling tube where the opacity measurement is made. While this method of exhaust opacity measurement does overcome some of the disadvantages of the full flow system in that alignment problems are eliminated, it also suffers from several drawbacks. Namely, the system is costly, it requires frequent maintenance, is subject to corrosion from sample condensation within the sampling tube and is awkward to use.
While each of these aforementioned exhaust plume opacity measurement systems has its individual advantages and disadvantages, they both suffer from the same general disadvantage namely, they require installation within the aggressive environment of the emission source itself and require the emissions source to be stationary in order to make an opacity measurement. For example, the current state-of-the-art method for measuring the opacity of exhaust emissions from diesel locomotives is the Wager opacity meter. In operation, the meter is attached to the locomotive smoke stack. In order to use this method for measuring diesel engine exhaust emissions, the locomotive must be moved onto a siding so that the instrument may be attached to the stack. For large nonstationary emission sources such as diesel locomotives this is undesirable. Not only is this process time consuming (generally requiring anywhere from 2-4 hours) but also it requires that the locomotive be withdrawn from service which is expensive and further, the opacity meter is subjected to the corrosive and erosive atmosphere in and near the smoke stack.
There have been numerous attempts to overcome the problems associated with measurement of the opacity of exhaust plumes set forth above. By way of example, U.S. Pat. No. 5,363,198 discloses a method for overcoming problems associated with full flow measurements of the opacity of exhaust plumes by employing two beams of light projected in two mutually perpendicular directions, thereby enabling the determination of the position of maximum opacity as well as the diameter of the plume at the point of maximum opacity. However, the apparatus must be mounted on the stack itself and the source must be stationary.
U.S. Pat. No. 4,647,780 discloses apparatus, wherein exhaust opacity is measured by drawing a sample of exhaust through a duct having a light source and detector located on opposite sides of the duct such that the light source directs a beam of light across the duct through the exhaust sample to the photodetector. The photodetector produces a signal whose strength is proportional to the intensity of the light beam transmitted through the exhaust sample. However, as set forth above, the apparatus requires frequent maintenance due to contamination of optical components and corrosion from sample condensation.
U.S. Pat. No. 4,432,649 recognizes the problems associated with contamination of optical surfaces associated with transmission measurements of the opacity of exhaust plumes such that false or incorrect measurements of opacity can be obtained. Disclosed therein is an apparatus for transmission measurements of stationary sources, wherein the signal transmission and receiving devices are accommodated within respective housings sealingly closed by windows which can be moved cyclically into and out of the light beam. In this way it is proposed that contamination of optical surfaces can be accounted for. However, there is no way to take into account refraction of the light beam by the hot exhaust gases, whereby the entire light beam can be shifted such that it is no longer centered on the detector and consequently, the entire light beam is no longer being measured.
What is needed is a method for monitoring exhaust emissions from large nonstationary sources, such as diesel locomotives, without moving them from the mainline track and without having to stop or slow the locomotive or train. Further, the monitoring method should be free from extraneous influences such as changes in optical properties of component parts and the need for extensive calibration procedures.
In order to make accurate measurements of an exhaust plume opacity of a moving emissions source several criteria must be considered. By way of example, the velocity of a locomotive moving down a mainline track and the dimensions of its exhaust stack fix the minimum time available for a measurement of the opacity of the exhaust plume at about 10 milliseconds. In order to be useful not only as an indicator of engine performance but also as a monitoring tool for regulatory agencies, it is necessary that the opacity of the exhaust plume be measured with an accuracy of .apprxeq.2% even in bright sunlight. The light source must be sufficiently intense to carry out an opacity measurement of this accuracy within the time scale set forth above, but not pose a safety hazard to personnel. Further, the light source must be sufficiently well collimated such that it can span the double tracks of a main railroad line (a distance of approximately 40 feet) and still be collected by a detector of reasonable size. The instrument must be sensitive enough such that an accurate measurement of total opacity of the exhaust plume can be made even if all the smoke is coming from one or two bad cylinders. The light beam from the light source must not be refracted by the hot exhaust gases to such a degree that it moves off the detector lens. It is desirable that the opacity meter only function during passage of an emissions source rather than continuously. Therefore, activation of the light source must be synchronized with the passage of a single or multiple locomotive(s) past the measurement point. The instrument must also be capable of measuring exhaust emissions from all the locomotives on multilocomotive train and must be able to correlate a given opacity measurement with a given locomotive. Finally, the instrument must be capable of withstanding the rigors of field operation.
Responsive to these needs, the present invention discloses an opacity meter that is capable of accurate measurement of the exhaust plume opacity of moving emissions sources, that is free from extraneous influences such as changes in optical properties of component parts and the need for extensive calibration procedures, that operates only during passage of an emissions source rather than continuously, that is capable of measuring exhaust emissions from all the locomotives on multilocomotive train and that can correlate a given opacity measurement with a given locomotive.